THE COMMON MUSSELS. 21 



forward along the left side of the pyloric caecum. From the base of 

 this artery, or from the anterior side of the aortic bulb, a small artery 

 runs forward on the right side of the pyloric csecum parallel to the 

 large gastro-intestinal artery. To the right of the gastro-intestinal 

 artery, and at the same level, the aortic bulb gives off: two small 

 arteries, one to the liver and one to the underside of the recurrent 

 intestine, but none of these is large enough to form a pair with the 

 gastro-intestinal artery, which is nearly median in position, and which 

 may therefore be called an unpaired artery. It is for a distance 

 nearly parallel to the aorta, and is in almost the same vertical plane 

 as the aorta, and sends off all along its course arteries to the hepatic 

 cseca and the portions of the intestine in its neighbourhood. It 

 curves down under the posterior side of the stomach, and becomes 

 coiled nearly horizontally with the cardiac csecum {g a, Fig. 24), 

 which its last branches supply. The gastro-intestinal artery is the 

 main artery that supples the stomach, intestine (anterior coil), and 

 liver. Its branches ramify through the liver, forming a pretty 

 preparation when injected. 



It will give a clearer idea of the arrangement of the arteries 

 to review them in connection with the different organs they supply; so 

 I shall describe them in this way. 



Arteries to the mantle, or pallial arteries. In M. latus the mantle 

 is supplied from two sources, the anterior aorta and the posterior 

 aorta. A line drawn perpendicularly to the long axis of the body 

 from the anterior end of the pericardial cavity would approximately 

 divide the regions of the mantle supplied from the two sources. I 

 shall speak of the part anterior to such a line as the anterior half of 

 the mantle, and of the part behind it as the posterior half. The 

 anterior halves of the mantle are mainly supplied by the ramifica- 

 tions of the anterior pallial arteries (a p a, Fig. 29), which pass for- 

 ward obliquely over the middle retractors. There are also some 

 small pallial arteries sent off into this region from the anterior aorta 

 and the anterior marginal arteries, near the anterior end of the 

 mantle. The posterior halves of the mantle are supplied below the 

 pericardial region by arteries given off from the posterior aorta 

 (p p a, Figs. 21 and 29) . The portions of the mantle below, behind, 

 and above the posterior adductor are supplied by branches from the 

 forks of the posterior aorta. The margins of the mantle-lobes 

 throughout are supplied by the circumpallial or marginal arteries 

 and their branches, which are short and sub-parallel, like the fibres of 

 the pallial muscles (a m a and jo m a, Fig. 29). 



